Aqueous paint has widely been used for coating industrial products. Aqueous paint is especially noted from a point of re-use without paint waste, because over-sprayed paint, i.e. aqueous paint that does not adhere to an article during coating thereof, is collected with water, filtered and condensed, and then re-used as paint. Aqueous paint also tends to be more widely used because a filtrate of the paint is water, and because the paint is easily handled without any problem, such as air pollution, in comparison with a paint using an organic solvent.
A recycling system for aqueous paint currently used is proposed in, for example, Japanese Kokai Publication Hei 5 (1993)-228422, of which a schematic view is shown in FIG. 4. In FIG. 4, when an article 250 is coated with aqueous paint 220 sprayed from a coating-gun 202 in a spray coating room 201, an over-sprayed paint 204 that does not adhere to the article is collected by water-curtain 203 formed from aqueous rinsing liquid. Liquid containing this collected paint is partially sent to an ultra-filtration apparatus 208 through a pre-filter 207 to separate the liquid into filtrate 213 and condensed paint 214. The filtrate 213 is used again as an aqueous rinsing liquid. The condensed paint 214 is subjected to condensation several times by the ultra-filtration apparatus 208. A small portion of the condensed paint 214 is sampled and subjected to compositional analysis so as to obtain by calculation an amount of each component to be supplied for forming recycled paint, so that a supplemental paint 209 is separately formed. The condensed paint 214 is then mixed with the supplemental paint 209 in a supplemental apparatus 210 to form an aqueous paint 211 for recycle use, which is spray coated onto another article.
After a certain period of operation of the above-mentioned recycling system, aqueous paint that has not been coated onto articles during a coating operation, and that spreads out to adhere onto a wall surface of the coating booth, is eventually dried and solidified so as to become floating matter or an impurity. In order to remove this floating matter or impurity from the recycling system, the coating booth is cleaned with water or booth circulating water 206 when a coating operation is stopped. This cleaning, in turn, significantly increases an amount of the booth circulating water 206 and results in extending a period of time for subsequent separation and condensation steps. If an amount of the booth circulating water 206 is too small, the floating matter or impurity is insufficiently removed and causes plugging of the pre-filter 207 and the ultra-filtration apparatus 208.
In the above mentioned Japanese Kokai Publication Hei 5 (1993)-228422, preparation of supplemental paint 209 supplements removed components such as floating matter or an impurity and supplies water and low molecular weight volatile materials. Preparation of the supplemental paint 209 is conducted by a compositional analysis of condensed paint 214, and a calculation of supplementing an amount of each component from a result of this analysis. Supplementing amounts obtained by the analysis does not make effective use of sources of paint, and it takes a relatively long time for preparing a recycled paint. In addition, this method does not control a liquid amount of a cleaning liquid and paint, and therefore, does not maintain material balance throughout the system.